How　to　design　a　sheet　pile　earth　retaining　wall　with　a　relieving　platform　is　still　a　challenging　topic　for　engineering　application,　and　there　are　some　problems　such　as　mechanical　characteristics　and　design　theories　lagging　behind　modern　structures.　The　optimization　of　the　unloading　board　parameters　and　pile　length　is　very　important　during　the　design　process.　In　this　study,　the　Finite　Difference　software　FLAC　was　used　to　develop　the　retaining　wall　numerical　analysis　model　based　on　the　size　of　model　test.　The　calculated　earth　pressure　by　using　numerical　simulation　method　was　very　close　to　the　result　of　model　test,　and　the　credibility　of　the　model　was　verified.　Fifty-nine　groups　of　numerical　simulation　tests　were　used　to　analyze　the　influence　of　the　mechanical　characteristic　of　retaining　walls,　such　as　its　width　and　buried　depth　of　unloading　board.　This　study　also　investigated　the　impact　of　pile　length　on　structural　failure　modes　and　its　stability.　It　was　found　that　when　the　buried　depth　of　unloading　board　is　0.30　similar　to　0.60　times　of　the　wall　height,　the　reasonable　board　width　is　0.38　similar　to　0.47　times　of　the　wall　height;　when　the　width　of　unloading　board　is　0.30　similar　to　0.50　times　of　the　wall　height,　the　reasonable　buried　depth　is　0.40　similar　to　0.59　times　of　the　wall　height.　Under　the　combined　conditions　of　the　reasonable　width　and　buried　depth,　the　reasonable　pile　length　is　the　wall　height.　Simultaneously,　three　kinds　of　failure　modes　of　retaining　wall　under　different　pile　lengths　are　proposed　to　achieve　the　stability　safety　factors　against　overturning　and　the　stability　of　retaining　wall.